Wait decorator to disallow re-entrantcy

Implemented `wait` decorator that waits for result when the function is called again without first result being available. Wrapping `_auth_me` to get auth token only once. This was required when `ImpersonatingAuthenticator.authenticate_tenant` is called many times when it is not already having a token. Using it in `CachingAuthenticator` to extract out the waiting pattern. 
No, _token definitely shouldn't be there for calculating the hash, since it would suddenly make the same object hash differently (if you had it as a dict key before the token got cached, then you wouldn't be able to look up the value again after authentication).